FIG. 1A illustrates a non-volatile resistive memory cell according to the related art. The memory cell illustrated in FIG. 1A includes a magnetic random access memory (MRAM) cell 10 that is made up of a hard ferromagnetic layer 15 that has a fixed magnetic orientation, a tunneling barrier layer 20, and a soft ferromagnetic layer 25. The magnetic orientation of the soft ferromagnetic layer 25 may be switched between a direction parallel to the magnetic orientation of the hard ferromagnetic layer 15 (the parallel state) and a direction anti-parallel with the magnetic orientation of the hard ferromagnetic layer 15 (the anti-parallel state).
The effective resistance of the MRAM cell 10 (i.e. the amount of current allowed to pass through the tunneling barrier layer 20) depends upon whether the MRAM cell 10 is in the parallel or the anti-parallel state. Further, when a voltage is applied across the MRAM cell 10, an MRAM cell 10 in either a high or a low effective resistance allows for either a high or a low amount of current to flow through the MRAM cell 10. Hence, the high and low effective resistances can represent a “1” and a “0” data bit, respectively, and these data bits may be read from the MRAM cell 10 by monitoring the amount of current that passes through the MRAM cell 10.
In FIG. 1A, a two-layer diode 30 is electrically connected to the MRAM cell 10 and both the two-layer diode 30 and the MRAM cell 10 are positioned between a first metal layer 35 and a second metal layer 40 The two-layer diode 30 includes a first p-doped semiconductor layer 45 and a first n-doped semiconductor layer 50. The two-layer diode 30 restricts current flow in one direction across the non-volatile resistive memory cell.
The solid curve shown in FIG. 1B illustrates the current that flows through the two-layer diode 30 as a variety of voltages are applied to the two-layer diode 30. At low voltages, and at voltages that place a reverse bias across the two-layer diode 30, little current flows through the two-layer diode 30. In contrast, when a forward bias is applied to and increased across the two-layer diode 30, the current flowing through the two-layer diode 30 also increases. Thus when the two-layer diode 30 is connected to an MRAM cell 10, a forward bias across the two-layer diode 30 allows current to flow through the MRAM cell 10. However, a reverse bias across the two-layer diode 30 passes less current and therefore electrically isolates the MRAM cell 10.
One disadvantage of using a two-layer diode 30 to electrically isolate an MRAM cell 10 is that the two-layer diode 30 nonetheless allows a significant amount of current to flow through the MRAM cell 10 when a reverse bias is applied. Another disadvantage is that a forward bias across the two-layer diode 30 allows current to increase relatively slowly as a function of added voltage.
A disadvantage of the device illustrated in FIG. 1A is the high voltage drop that occurs across the two-layer diode 30 when a forward biasing voltage is applied across the non-volatile resistive memory cell. Further, since both the forward bias voltage and the series resistance of the two-layer diode 30 are strong functions of current, there may be a wide range of two-layer diode 30 forward voltage drops caused as a wide range of forward currents are applied to the two-layer diode 30. Hence, it becomes difficult to deconvolute the voltage drop due to the presence of the two-layer diode 30 from the voltage drop due to the MRAM cell 10 and data bits are not read as easily.
In addition, the two-layer diode 30 illustrated in FIG. 1A is typically a thin-film diode and therefore may leak a substantial amount of current. This phenomenon is partially illustrated in the lower, left quadrant of the graph in FIG. 1B, where reverse biasing across the two-layer diode 30 nonetheless allows some current flow. When many devices such as the device illustrated in FIG. 1A are electrically connected together in an array, diode leakage currents from many other devices in the array (that are electrically connected in parallel with the selected device) limit the signal resolution and increase noise when detecting data stored on the selected device.